Because of the various physical and chemical properties of diamond, diamond coatings have great potential for utilization in a wide variety of industries. In particular, diamond has a very high thermal conductivity and a very low electrical conductivity. It is very hard, inert and chemically resistant. Thus, it is a particularly effective protective coating for the wear or work surfaces of items such as cutting tools, drills or bearings; and, it is an effective protective coating for example for medical or surgical instruments or implants etc. In Spear, "Diamond-Ceramic Coating of the Future", Journal of the American Ceramics Society, Vol. 72, No. 2 (Feb. 1989), pages 184-187 incorporated herein by reference, desirable properties and applications of diamond coatings are described in considerable detail.
Diamond coatings have been applied in a wide variety of manners. One of the older and best known methods of creating a diamond coating is through a sintered carbide process. While a great many variations in such processing are known, a typical examples of such processing are provided in the descriptions in Levin, E, et al, "Solid State Bonding of Diamond to Nichrome and Co. - 20% wt W. Alloys", J. Mater. Sci. Lett.; Vol. 9 (1990) p. 726; and, Budinski, K.G., Engineering Materials, p. 128-132, both of which are incorporated herein by reference.
In general, a carbide sintering process for preparation of diamond coating concerns provision of powdered carbide of appropriate size (typically about 1-100 microns). The carbide is sintered with a binder onto a substrate under conditions of high pressure and temperature. Typically the binders are metal binders such as nickel or chromium. Typical substrates are steel, ceramics or similar materials. Typical conditions for application to the substrate are about 3.0 GPa pressure and 25.degree. to 1,300.degree. C.
While such processes have been effectively utilized to provide diamond coated substrates, there are some characteristics of the process, and products resulting from the process, which are less than preferable, but which generally result if the process is followed.
For example, products made by a sintering process comprise substrates with a diamond coating that has regions of exposed binder between diamond particles. While diamond is very hard, inert, and resistant to both chemical and physical deterioration, typical binders do not possess such characteristics (at least to the extent of diamond). Thus, the binder represents a site at which corrosion or other attack on the coating can take place, limiting the coating lifetime and increasing likelihood of failure. Also, the binder may not possess the desireable insulation properties associated with diamond.
If the coating is of a substrate that is to be utilized under conditions of extreme wear or pressure, such as in a cutting tool, drill bit or bearing, problems with wear over the lifetime of such coatings can arise from the fact that in general the regions of binder between the diamond particles represent relatively soft material subject to contact wear at a relatively high rate, by comparison to the diamond particles themselves. That is, binder is undesirable with respect to wear.
Another characteristic of diamond coatings formed from sintered diamond particles with binder is that there tends to be a relatively wide range (1-100 microns) of diamond particle sizes present in the diamond powder utilized for the formation of such coatings. Under the stresses to which coated articles are placed during use, different particle sizes will tend to be affected in different manners. Thus, uneven wear in the diamond coating can result. In addition, uneven action (grinding, cutting, etc.) as a result of the different particle sizes may occur.
As alternatives to sintering processes, chemical vapor deposition (CVD) techniques have been developed for preparing diamond coatings. A variety of such techniques have been reported. In general they concern heating a mixture of hydrogen and methane to a sufficient temperature to achieve a desired disassociation into hydrogen atoms and fragments of organic material. From the highly excited mixture, a deposition of diamond can be obtained, onto a cooler substrate surface. In one reported method, a diamond coating is provided by means of a chemical vapor deposition by passage of hydrogen and methane over a heated tungsten filament. In certain other methods, a nonequilibrium, low pressure plasma plume or stream is generated from hydrogen gas and methane, with the deposition occurring from the plasma plume. See Derjaguin, B. V. et al, "The Synthesis of Diamond at Low Pressure", SciAm., 233(5), p. 102-109 (1975), incorporated herein by reference.
From a typical chemical vapor deposition, the resulting diamond coating is joined to or in communication with the substrate. This provides a relatively rigid diamond coating, somewhat more susceptible to damage from stresses of thermal shock or physical shock in use, than a coating made according to the previously described sintering process. A reason for this is that the binder, which is between the diamond particles in a sintered film, can absorb (or dampen) both thermal and physical shock.
A characteristic feature of many reported chemical vapor deposition procedures used for the preparation of diamond coatings, is that they are relatively slow. For example, typically the diamond is deposited at rates on the order of about 0.1 microns per hour from a tungsten filament process. If a diamond coating on the order of about 5-20 microns thick is desired, relatively long deposition times are contemplated. Thus the technique is relatively expensive and inconvenient. For a general discussion of conventional CVD processes see Yarbrough, W.A. et al, "Current Issues and Problems in the Chemical Vapor Deposition of Diamond", Science, Vol. 247, p. 688-696 (Feb. 1990), incorporated herein by reference.
In addition, while the diamond coating being prepared is relatively stable to the high temperature conditions of the process, the substrate may not be. Under exposure to relatively high temperatures for extended lengths of time, redistribution in the substrate, resulting in unwanted phases or properties at the interfaces, may occur. That is, the substrate may become "over tempered." To accommodate for this, in some systems chemical vapor deposition has been applied to tungsten carbide substrates which, although relatively expensive, are quite heat stable. When it is desired to apply a diamond coating to less expensive "tool steel", a deposition rate of about 0.1 microns per hour is sufficiently slow to ensure that over tempering of the substrate will occur in many instances, resulting in undesired potential in the substrate for flaw or failure.
As indicated above, chemical vapor deposition techniques have been developed wherein nonequilibrium, low pressure, plasma is generated from the hydrogen and carbon feed-stock, with the deposition occurring from the plasma flame or stream. Typically such low pressure processes are operated with a plasma stream pressure on the order of about 10.sup.-2 Torr to about 10 Torr. Under such relatively low pressures, the electrons in the plasma stream are quite hot, by comparison to the carbon or organic fragments therein.
Such techniques (deposition from nonequilibrium, low pressure plasma) have been used to apply coatings at deposition rates on the order of about 1-5 microns per hour. Thus, they are relatively rapid by comparison to the tungsten filament vapor deposition technique. Such rates are undesirably slow, however, for utilization to apply relatively thick coatings (at least about 15 .mu.m) on tool steel substrates and the like, wherein tempering usually occurs at a rate of about 2.5 cm of depth per hour.
A variety of techniques have been utilized to generate the plasma streams for such chemical vapor depositions. These include microwave generation, RF glow discharge, and DC glow discharge.